Recently, in an electric automobile or a so-called hybrid car in which a motor and an engine are commonly used, it has been discussed not to use an expensive permanent magnet, but to use an SR motor that has a simple and rigid structure, and excels in a high speed rotation and an environmental resistance. Generally, the SR motor has a construction in which a rotor made of magnetic steel sheet is disposed rotatably coaxially with a stator inside the stator in which a coil is wound around. Salient poles projecting inward are formed on an inner peripheral side of the stator, and a coil is wound around on this salient pole to form a winding. Salient poles projecting outward are formed radially on an outer periphery of the rotor, and adapted to approach, oppose and separate from the salient poles of the stator in association with the rotation of the rotor. The salient poles of the rotor and the salient poles of the stator are set to even numbers so that both the salient poles of the rotor and the salient poles of stator do not have a multiple relation with each other with the result that when certain salient poles of the stator and the rotor are opposed, the other salient poles of the stator and the rotor are deviated at the positions. That is, when the number of salient poles of the rotor is, for example, four, the number of salient poles of the stator is set to six, and when the number of salient poles of the rotor is six, the number of salient poles of the stator is set to eight.
In such an SR motor, when current flows, for example, to a pair of the opposed windings of the stator, a magnetic flux directed from the salient pole of the stator toward the salient pole of the rotor is generated. Thus, the salient pole of the rotor is attracted to the salient pole of the stator to generate a torque at the rotor. As described above, the salient poles of the stator and the rotor are set so that when certain salient poles are opposed, a deviation arises at the other salient poles. Therefore, the winding of the salient pole set to the state that the other salient poles are deviated, is applied with power, the salient poles of the deviated state are attracted, and thus the rotor is rotated. When this operation is continuously conducted, the salient poles of the rotor are attracted to the salient poles of the stator continuously, and the rotor is rotated around its axis.
On the other hand, such an SR motor can be used as a generator, for example, Japanese Patent Application Laid-Open Publication No. 2001-57795 and Japanese Patent Application Laid-Open Publication No. 2001-78490, disclose a control system for preventing overcurrent at a power generation time and thereby generating electricity in a high efficiency. In the Publication No. 2001-57795, a maximum amount of current, when a power regeneration is conducted at present time, is predicted and calculated based on a rotational speed of the rotor or on an amount of supply current during a supply mode for using the SR motor as a motor by supplying a power from a battery. When the maximum amount of current reaches a predetermined rate, a regenerative mode in which an electromotive force generated in the winding is recovered in the battery is performed.
In the Publication No. 2001-78490, in addition to the above-mentioned supply mode and regenerative mode, a reflux mode in which both ends of the winding are set to the same potential, is set. At the reflux mode time, the winding is short circuited, and hence winding current increases. A winding current value is always monitored. After the supply mode is switched to the regenerative mode, and when the winding current value reaches a lower limit value, the regenerative mode is switched to the above-mentioned reflux mode. In the reflux mode, a current value rises, and when the current value reaches an upper limit value, the reflux mode is again switched to the regenerative mode. This switching of the regenerative mode and the reflux mode is continued until the rotor becomes a predetermined rotary angle. Thus, the winding current value is controlled between the upper limit value and the lower limit value so as to prevent the winding current from becoming large in a projecting manner.
Meanwhile, in the above-mentioned control system, in an inductance reducing region (dL/dθ<0), the current is controlled to fall within a predetermined range by means of the alternating mode and the reflux mode, and thereafter regenerative current flows. Since the current flowing in the range of dL/dθ<0 affects a braking force to the rotor, it is necessary to suppress a control current value in the alternating/reflux mode to raise a power generating efficiency. However, when this control current value is suppressed to a small value, the regenerative current value is also reduced, and hence there is a problem that it becomes difficult to assure a necessary amount of power generation. In order to control a motor current, a sensor for detecting the current value and a feedback control circuit which is operated at a high speed are necessary. Hence, there is a problem that the apparatus becomes expensive. An object of the present invention is to improve a power generating efficiency of a generator having an SR motor structure.